Kaolinite [$Al_2Si_2O_5(OH)_4$] was successfully synthesized by a hydrothermal process from amorphous $Al(OH)_3$ and $SiO_2$ at $230^{circ}C$ under the pressure of $30 kg/cm^2$. The experiments were performed varying temperatures ($180{sim}280^{circ}C$), pressure ($10{sim}60kg/cm^2$), chemistry ($Al_2O_3/SiO_2 = 0.5{sim}0.38$) and pH ($0.3{sim}9.5$) of the solution. The autoclaving was carried out in a closed stainless steel vessel. Kaolinite appears from the starting composition of $Al_2O_3/SiO_2= 0.5$ with boehmite and was stable as a single phase with the composition of $Al_2O_3/SiO_2=0.45$. Boehmite was a stable phase below $200^{circ}C$ for the 240 h period of autoclaving, but kaolinite appeared even in 20 h at $230^{circ}C$. The single kaolinite phase of a good crystallinity was observed at pH ranging 2 to 6. That indicates that pH is one of the most critical parameters for the successful formation of kaolinite. The optimal molar ratio of $Al_2O_3$ to $SiO_2$ was determined to be 0.45. The XRD pattern of the synthesized kaolinite coincided with that of natural one and its morphology was the cluster type of the kaolinite crystals (diameter = ${sim}3{mu}m$), irrespective of starting material, composition and temperature.